This invention relates generally to terrain aided navigation systems, and more specifically, to methods and systems for interferometric cross track phase calibration.
A precision terrain aided navigation (PTAN) system is an autonomous navigation aide that measures terrain features using radar and correlates those terrain features to stored digital terrain elevation data (DTED) to provide a precise air vehicle position. In operation, a PTAN Doppler interferometric radar system incorporates beam sharpening to measure cross track ranges. More specifically, the range to the highest terrain in the ground swath is measured as well as the angle from the air vehicle to the highest terrain point. With this data, a distance to a region of interest can be determined. One known PTAN system relies on multiple radar receiver channels and corresponding antennas which provides for reception of ground return signals and a determination of phase differences between the ground return signals received by each of the antennas.
To prevent ambiguous cross track range determinations because of the possibility that the ground return signals received by each of the antennas may include a phase shift of greater than 360 degrees, three receivers and antennas are used in one known PTAN system. The three antenna and receiver system provides a mechanism to assure that the phase difference determinations are unambiguous. Such a process is sometimes referred to as unwrapping of the phases.
With this PTAN system, cross track ranges can be calculated to various types of terrain very precisely. However, the PTAN system may also provide erroneous range and angle data if the signal delay through one channel varies relative to the signal delays through one or more of the other two channels. One known way to prevent or reduce such errors is a time consuming, complex, and costly calibration process that is conducted as part of a PTAN system testing process that is performed just before shipment of the PTAN system. However, this calibration process does not prevent or take into account variances in channel delays that might occur during field operation over time due to aging of the individual components utilized within the system, or due to environmental variances, such as temperature, that the PTAN system may encounter.